I'd call
Susannah Sukie or
Susie or straight out
Susannah, although
Annie is cute for someone else.
I definitely prefer this spelling, it somehow seems quite a lot grander than
Susanna.
I tend to shy away from trying to describe the bearers of names, but I do know a really lovely girl called
Susannah-- I think a
Susannah can only ever be a really lovely person.
I think it works splendidly in a sibset with
Frederick. :D